In the production of motor vehicles sheets, metallic substrates are frequently coated to achieve certain characteristics, for example good corrosion resistance. A coating is usually applied strip-wise or sheet-wise to the metallic substrate before a forming process, so that not only the metallic substrate but also its coating can have good forming characteristics. A typical example of such coating is hot-dip galvanizing of steel parts, which are used for example for the body shell of a motor car. The coating quality is therein dependent on various parameters, therefore it is desirable in the preliminary stage, i.e., before the subsequent processing of the metallic substrate for example into body parts, to test the surface quality. A similar problem also concerns uncoated sheets, for example thin sheets, since for example rolling defects frequently only appear when the thin sheet is formed into a product. In this case it is also desirable to test the surface quality, while taking the further forming of the sheet into consideration.
A plurality of methods for testing the surface finish of a metallic substrate is well-known from the art. For example, it is known from DE 101 11 296 A1 to examine the surface quality of a flat product by evaluating electronic images of the surface of the flat product. However, information about whether and for which forming processes the surface and/or the coating are suitable cannot be obtained with the known method.